The present invention relates to a screw used in an accumulator and the like. In particular, it relates to a pitch diameter displaced screw in which, between an external screw and an internal screw that engage with each other, the strength of a weaker screw is improved.
Conventionally, for the accumulator and the like, there is used a triangular screw, for example, a metric screw and an inverse buttress screw. As shown in FIG. 4, in a standard triangular screw, a sectional shape of a thread ridge is formed in an approximately regular triangle. Further, as shown in FIG. 5, in a standard inverse buttress screw, flank inclination angles of its thread ridge are formed in an inverse manner to that of a thread ridge of a buttress screw, i.e., an inclination angle of a pressure flank is formed greater than that of a clearance flank, as described in Japanese unexamined utility model publication No. Hei 4-77017 (1992-77017).)
In FIGS. 4 and 5, numeral 1 denotes an internal screw meshing with an external screw 2, and 4 denotes a thread ridge of the internal screw 1 having a pressure flank and a clearance flank. Numeral 6 denotes a pitch diameter of the internal screw 1, i.e., a diameter of such an imaginary cylinder that a width P1 of the thread ridge 4 becomes equal to a width P2 of a thread groove 8. Numeral 9 denotes an inner diameter of the internal screw 1, and numeral 11 denotes a thread groove of the external screw 2, meshing with the thread ridge 4 of the internal thread. Numeral 13 denotes an outer diameter of a thread ridge 14 of the external screw 2, numeral 16 denotes a pitch diameter of the external screw 2, and A2 denotes a load applying direction.
Incidentally, a pitch P is equal to a total of the width P1 of the thread ridge 4 and the width P2 of the thread groove 8. Further, each of the widths P1, P2 is a half of the pitch P, i.e., P/2.
In the conventional screw, as to the mutually meshing external screw 2 and internal screw 1, since the pitch diameters 6, 16 are located in the center of meshing heights of the thread ridges 4, 14, the width P1 of the thread ridge 4 of the internal screw 1 becomes equal to the width P2 of the thread ridge 14 of the external screw 2, meshing with the thread groove 8 of the internal screw. When a load in the arrow A2 direction is exerted on the external screw 2 in this state, a compressive force is applied to the external screw 2 while a tensile force is applied to the internal screw 1, and both forces are equal. Incidentally, at this time, a compressive stress concentrates on thread bottom 10 of the external screw 2, and a tensile stress concentrates on thread bottom 5 of the internal screw 1.
However, since a screw has a property that it is resistant to the compressive force but weak at the tensile force, in a case where the force of the same magnitude is applied to both thread ridges 4, 14, a screw to which the tensile force is applied, i.e., the internal screw 1, is the first to suffer a breakage from the thread bottom 5. Namely, the internal screw 1 is smaller in its fracture strength than the external screw 2, and weaker.